Project Summary Parkinson's disease (PD) is a progressive neurodegenerative movement disorder caused primarily by the degeneration of dopaminergic neurons in the substantia nigra. Current therapies for PD are palliative but no disease-modifying therapies exist today. Mutations in the VPS35 (PARK17) gene were recently identified as a cause of late-onset, autosomal dominant PD, with a single mutation (D620N) detected in PD individuals and families worldwide. How mutations in VPS35 precipitate dopaminergic neurodegeneration in PD remains obscure. It is critical to identify the molecular and cellular mechanisms that lead to neurodegeneration due to VPS35 mutations in order to understand the pathophysiology of PD and develop new therapeutic strategies. VPS35 is a core component of the retromer complex responsible for the recognition and sorting of transmembrane protein cargo from endosomes to the Golgi network or plasma membrane for recycling. How familial mutations influence VPS35 retromer function in PD-relevant neuronal populations and animal models is not known. We have recently developed a novel viral-mediated gene transfer model of PD in rats where the overexpression of human D620N VPS35 induces the degeneration of nigrostriatal pathway dopaminergic neurons, thereby formally establishing a pathogenic role for the D620N mutation in vivo. In the present application, we now propose to extend our studies to novel D620N VPS35 knockin mice with physiological levels of VPS35 expression as a new relevant model of VPS35-linked PD (Aim 1). We propose to identify neurodegenerative phenotypes in the D620N VPS35 knockin mice, including the development and progression of dopaminergic neuronal and axonal degeneration, striatal catecholamine and motoric deficits, neuropathology and protein aggregation. We will evaluate abnormal retromer cargo sorting and VPS35 protein interactions in brain tissue from these mice to identify molecular mechanisms underlying the D620N mutation. We will also evaluate a novel interaction of VPS35 with ?-synuclein and LRRK2 in rodent models of PD to determine whether these proteins converge in common pathogenic pathways underlying neurodegeneration in PD (Aims 2-3). We will determine whether VPS35 overexpression or pathogenic mutations can protect or exacerbate ?Syn-dependent neurodegeneration, respectively, in two well-characterized rodent models of PD (Aim 2). We will also provide evidence of whether familial LRRK2 mutations act to induce a retromer deficiency in the brain using two distinct LRRK2 rodent models of PD, and we will evaluate whether VPS35 mutations or deficiency in mice can exacerbate mutant LRRK2-induced dopaminergic neurodegeneration (Aim 3). Our comprehensive proposal is novel, innovative and timely and will provide important insight into the pathogenic actions and mechanisms of VPS35 mutations in PD by using a novel knockin mouse model.